In recent years, a wearable terminal has been developed that is worn constantly by a user, enabling recording everyday life experiences of the user as a lifelog. Here, the wearable terminal refers to a compact terminal that can be worn on the body of the user. The present invention focuses on a terminal that includes video and audio recording functions so that the wearable terminal can store audio and video. The wearable terminal continues the recording function even without any explicit operation, namely an operation performed by a hand or a finger, being performed. Also, the wearable terminal includes an attachment unit, and is a portable terminal or portable audio/video recording device that can be affixed to clothing or, by attaching a strap to the attachment unit, supported at a predetermined reference position of the body, e.g. hanging from the neck. When the sound pickup direction of a microphone attached to this type of wearable terminal faces a front direction faced by a camera, the microphone can pick up a voice, etc., of a person talking to the user face-to-face, and when the sound pickup direction faces upward, the microphone can pick up the voice, etc. of the user. Since wearable terminals used for this purpose are required to record sound clearly even in a noisy outdoor environment, a directional microphone such as a unidirectional microphone is used to sensitively pick up acoustic signals from a specified direction.    Patent document 1: Japanese Patent Application Publication No. H01-39193    Patent document 2: Japanese Patent Application Publication No. 2005-37273
However, though the sensitivity of a unidirectional microphone is high in a specified direction, the sensitivity is low in other directions. Therefore, there is a problem when the sound pickup direction changes due to motion, e.g. when a user wearing the wearable terminal is walking. FIGS. 1A and 1B show directivity characteristic patterns pertaining to sensitivity of a unidirectional microphone and an omnidirectional microphone. FIGS. 1A and 1B show that although the omnidirectional microphone picks up sound from all directions with equal sensitivity, the unidirectional microphone picks up sound sensitively from the front direction while sound from other directions is suppressed. Accordingly, for example, when the wearable terminal is hanging from the neck by a strap, and the microphone is facing front to pick up the voice of a person to whom the user is talking, if a movement of the user causes the neck strap to twist and the wearable terminal to rotate 90° to the right of the front direction, sound is suppressed from the front direction that was originally intended to be the sound pickup direction, and sound from the direction 90° to the right, which was originally intended to be suppressed, is picked up with high sensitivity.
Also, the unidirectional microphone is vulnerable to noise. FIG. 2 shows frequency characteristics pertaining to sensitivity of the unidirectional microphone and the omnidirectional microphone. The unidirectional microphone is realized by synthesis of two omnidirectional microphones, which are arranged at a distance d apart from each other. A phase difference is given to the signal picked up by one of the omnidirectional microphones, and the output of one of the omnidirectional microphones is subtracted from the output of the other omnidirectional microphone. This synthesis method is called a sound pressure gradient-type directivity synthesis method. FIG. 2 compares the sensitivity of the omnidirectional microphones before the synthesis and the unidirectional microphone after the synthesis. In the high frequency area, both the unidirectional microphone and the omnidirectional microphones demonstrate a favorable sensitivity even in the presence of noise. However, while the sensitivity of the omnidirectional microphones is only slightly dependent on frequency, the sensitivity of the unidirectional microphone significantly decreases at low frequencies. In particular, as d, which is the parameter representing the size of the unidirectional microphone, becomes smaller, low-frequency sensitivity decreases. Since a portable device such as the wearable terminal is required to have a small size, overcoming the sensitivity problem by arranging the microphones farther apart is difficult. The signal to noise ratio of the unidirectional microphone becomes smaller at lower frequencies. Since noise generated by the movement of the user has a low frequency of several Hz, when the sensitivity of the unidirectional microphone is corrected by amplifying the low frequency area with use of an equalizer, the low-frequency noise component is relatively emphasized.
Patent document 1 discloses conventional technology pertaining to noise resistance measures in a unidirectional microphone. Patent document 1 discloses a device that switches between a unidirectional microphone and an omnidirectional microphone in accordance with a result of detecting wind noise in an acoustic signal picked up by a microphone occurring when wind hits the microphone. However, though the device of patent document 1 has a structure suited for achieving the aim of suppressing wind noise in a unidirectional microphone, sensing noise that occurs suddenly due to motion of the device and switching appropriately between output signals of two microphones is difficult.
Since the wearable terminal is worn constantly and sound pickup continues independently of the status of the user, there is a constant risk of the movements of the user causing the movable terminal to be moved or to collide with the body of the user. When using a unidirectional microphone, motion-related noise and the influence of a shift of the sound pickup direction significantly reduce sound pickup quality. Therefore, measures to counter such effects are necessary.
An aim of the present invention is to provide a device, such as a wearable terminal, that continuously performs sound pickup in an unstable environment and can prevent a reduction in sound quality as much as possible even when the device is in motion.